A beginner's guide to buying a mountain bike

This is a guideline for a buying a mountain bike. I consider both
off-road riding and riding on paved surfaces. It doesn't suggest any
particular bikes, but instead explains what you should be looking for,
and it will allow you to choose a bike in your price range that is
good quality, appropriate for the kind of riding you want to do, and
fits you correctly.

I assume that you want something that isn't too expensive (since
you are a beginner and aren't sure how much you will end up riding it)
nor too cheap (since you want a decent-quality bike that will keep you
happy for awhile). Thus I'm only considering bikes costing something
between about 250 to 500 pounds.